1. Field of the Invention
The present invention relates generally to gates for controlling traffic through a fence or along a roadway, and has particular reference to lift gates and parallelogram lift gates.
2. Background of the Prior Art
Gates having closure members, such as rails, pivotally connected to a vertical pivot post for movement in a horizontal plane require sufficient unobstructed ground area to accommodate the swing radius of the closure members. Lift gates having closure members that pivot through a vertical plane have been widely used instead wherever the ground area devoted to a gate was to be minimized, thereby permitting use in cramped areas. Such lift gates included, for example, the safety gate assembly disclosed by Naegelli et al., U.S. Pat. No. 5,299,386; the security gate disclosed by Osborn, U.S. Pat. No. 5,263,281; the parking gate disclosed by DeWitt III, U.S. Pat. No. 5,136,810; the portable traffic control gate disclosed by Wagner et al., U.S. Pat. No. 4,681,479; and the gate with multiple pivot axes for rail members disclosed by Porter, U.S. Pat. No. 4,519,164.
A type of lift gate known as a parallelogram gate has also been widely used. The closure members of a parallelogram gate comprise parallel upper and lower rails pivotally mounted for movement in a vertical plane, and a plurality of vertical members disposed along the lengths of the rails, the upper and lower ends of the vertical members being pivotally attached to the upper and lower rails, respectively. See, for example, the swinging lift gate disclosed by Carr, U.S. Pat. No. 4,658,543. As the gate is raised towards vertical, the upper and lower rails of a parallelogram gate move closer together, thereby closing the parallelogram and providing maximum clearance for vehicles to pass by the gate. The ability to close the parallelogram also permits compact shipment of the closure members of a parallelogram lift gate.
As the closure members of a lift gate are raised, the gate, and particularly the pivot points of the closure members located on the pivot post of the gate, tends to become maximally stressed by any wind forces impacting the closure members. It appears that prior to the present invention, no provision was made in lift gates for adequately distributing and dissipating such forces at the pivot point(s) so as to stabilize the gate against wind loads. There remains, therefore, a need for a parallelogram lift gate capable of adequately distributing and dissipating wind load forces.